e2image: write progress information to stderr

e2image: write progress information to stderr

This allows "e2image -rp /dev/sdc1 - | bzip2 > sdc1.img.bz2" to work
correctly, so the progress information doesn't corrupt the image being
sent to stdout.

Also add a diagnostic indicating that the -p option is currently only
implemented for raw mode.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>